Since one of the errormessages states that it's automatically creating a math environment/symbol i guess my problem is similar, though the answer to the question is just codesalad i don't understand and thus can't really use.

Any explained solution to either prevent LaTex to create a math-environment or getting a grey background under the multicol is highly appreciated.

Btw. i just started using LaTeX, so explanations are also highly appreciated.

I just have one question left: which value does the \dimexpr... command return? And just to confirm that i understand it right: the \parbox creates a vertical box environment, which gives \multicols the environment it needs, right?
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ChrisNov 22 '12 at 10:10

it is an (etex but supported in all current distributuons) syntax which expands to the value given using infix syntax (an alternative to using the calc package) you want a full width box but the \parbox can not be \textwidth wide as \colorbox like \fbox adds padding all round so there is an additional \fboxsep space added to each side that you need to subtract to get the size needed for the inner box.
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David CarlisleNov 22 '12 at 10:12